This invention relates to an apparatus for and a method of making a hard copy having a predetermined pattern and more particularly to an automated apparatus for and a method of fabricating a printed wiring board (PWB) by a projection imaging process.
Wiring circuits which are used in electrical assemblies are currently mass-produced by fabricating printed wiring boards incorporating a desired wiring circuit pattern. One approach to fabricating printed wiring boards is by depositing a coating onto a double-sided copper clad substrate in the form of a desired wiring circuit and placing such a substrate in an etching solution to dissolve the copper away except where it is protected by the coating. What is left is a copper pattern in the form of the desired wiring circuit. Holes are then drilled in the board and electrical connections are formed between the opposing surfaces of the substrate by copper plating. Alternatively, the holes may be drilled prior to the deposition of the patterned coating.
At present, the transfer of the wiring image to the substrate is done in one of the following two ways. The older way is to use the designer's taped wiring pattern to create a screen which, when laid over a copper clad substrate, acts to allow a coating material to transfer to the substrate only where the pattern originally existed. Until very recently, this was done totally manually. Recently, automated screening machines are coming to be used but various problems are experienced with this method. For example, the screen by this method requires frequent cleaning and this affects the production efficiency adversely. Also, the results obtained with this method are highly dependent on the operator, and the transferred pattern generally is not sufficiently sharp to satisfy the design requirement, i.e., the dimensions of the lines and spaces which should be less than 0.008-0.01 inches.
According to the newer approach to the fabrication of hard copies such as printed wiring boards, a wiring pattern image called a "phototool" is made on a photoplotter from data generated on a computer-aided design system. In this process, a printed wiring board substrate is coated with a solid photoresist film which adheres to the substrate and undergoes a polymerization process when exposed to controlled ultraviolet or visible light. A working copy of the phototool is positioned on top of the resist to block the light, thereby preventing polymerization of the resist beneath the wiring pattern image. The printed wiring board is then inserted into a solution that dissolves the polymerized photoresist, leaving a resist having a pattern similar to the wiring circuit pattern. This photoresist method can yield very fine lines and spaces down to 0.001-0.003 inches, but it is mostly a manual process and production rates are thus limited. Yields are also limited due to the intrinsic nature of hardcopy phototools, i.e., the tools may have defects due to their initial fabrication process and typically acquire additional defects with time and use due to dust and other particulates which settle on them and also due to damage created by handling as well as various contact and near contact printing operations requiring them to touch another surface.
Recent attempts to automate the exposure of the printed circuit board for the optical transfer of wiring pattern include the method of imaging directly onto the substrate with a finely focused ultraviolet (UV) or visible laser beam. The laser beam is actuated to move laterally across the face of the printed wiring board substrate by the use of mirrors, while the board itself moves longitudinally under the laser light by means of a moving table, thereby implementing a raster scan. By this method, however, the speed of the exposure, or equivalently the speed of optical transfer of the wiring pattern onto the resist, is limited by the serial nature of this exposure process and the intensity of the laser light which must directly polymerize the resist. Fundamental problems still exist with this process, such as the necessity to convert the original vector-formatted wiring pattern into a raster format, which requires considerable time and computer power.
Another fundamental problem is the inability to reproduce straight line edges for features which do not run parallel or perpendicular to the raster pattern. The sampling nature of raster imaging typically results in jagged line edges often referred to as "the jaggies" for such off-axis features. Additional fundamental problems with laser exposure system are reciprocity and nonlinearity due to the inherent high peak exposure intensities and interference effects due to use of coherent laser light. Furthermore, direct laser exposure systems operating in a raster mode have only one degree of modulation freedom, exposure intensity. Thus, feature resolution and exposure intensity cannot be independently controlled and optimized.
In addition, it is necessary to redraw the pattern with the laser for every new board that is being fabricated, which requires stringent precision of the mechanical components used for mounting the printed wiring board and scanning the laser. Furthermore, the process is only applicable to one side of the board at a time, since the mechanism to be used to translate the printed wiring board and to hold it flat precludes exposing both sides of the board simultaneously.
The PWB fabrication approach described above is typically referred to as "subtractive" because copper is subtracted from the board. It is known as a "print and etch" method because the etch-resistant coating is printed, followed by etching of the copper. An alternative "additive" PWB fabrication process involves plating. Here, copper is deposited on the board to create the wiring patterns. Both "additive" and "subtractive" PWB fabrication processes require creation of the wiring circuit pattern by a photolithographic process. For "subtractive" processes using negative working (photopolymerized) resists, the final wiring pattern will correspond to bright lines or patterns on an otherwise dark background. For "additive" processes using negative working resists, the final wiring patterns will correspond to dark lines or patterns on an otherwise bright background. The resolution, or ability to resolve fine details, is typically not as good for negative resists as for positive resists, i.e., resists in which the unexposed region remains after development and the exposed region is developed away. Thus, in some applications, it is desirable to use positive resists. The required exposure patterns for "additive" and "subtractive" PWB fabrication processes and negative and positive photoresists are summarized in Table 1.
TABLE 1 ______________________________________ PWB Required Required Process Resist Exposure for Exposure Type Type Wiring Pattern Background ______________________________________ subtractive negative bright dark additive negative dark bright subtractive positive dark bright additive positive bright dark ______________________________________
Thus, in order to be applicable to both subtractive and additive PWB fabrication processes and to enable use of either positive or negative photoresists, it is required that the PWB exposure system be able to generate both bright-on-dark as well as dark-on-bright images of the desired wiring pattern.
It has been known to alternatively use thermally addressed liquid crystal (TALC) light valves for projection imaging as described, for example, by Robert A. Heinz and Robert C. Oehrle in "Rapid Generation of Complex Images with a Liquid Crystal" in the Western Electric Engineer (April, 1977). The authors, however, recognized that the use of TALCs introduces significant problems during implementation of such a system. Although Heinz and Oehrle speculated on the possibility of applying TALCs to direct projection printing on PWBs, they never showed a single pattern printed on a photoresist with a TALC nor did they show a PWB fabricated with a TALC. One of the problems is the lack of adequate contrast ratio because TALCs are not capable of high contrast ratios such as on the order of 1000:1 which was then considered absolutely necessary. Heinz' and Oehrle's own results showed a contrast ratio of 20:1 for an image resolution of 30 lines per inch on a TALC cell. Thus, even a 20:1 contrast ratio (50 times lower than that thought to be required) would require a 10 inch.times.10 inch active liquid crystal area for a PWB with 8000 features per linear dimension (board width divided by minimum feature size). Also, the application of ultraviolet light is problematical because it is difficult to make a transmission cell which transmits ultraviolet light, and if such a cell were used in the aforementioned authors' system, it would be rapidly degraded. If visible light is used, on the other hand, an arc lamp operating at 1 kW or more is needed in view of the resists in order to keep the exposure time reasonably short. This would then present a heating problem due to absorption of the light by residual absorption in the transmission cell. Furthermore, complex optics are required with the method with a transmission cell to enable writing and projection of the cell image simultaneously. Finally, the systems proposed by Heinz and Oehrle and by Klaiber (U.S. Pat. No. 4,014,466 issued March, 1977) would have inadequate production throughputs because they used a single cell and cell position for writing PWBs and because in order to create a bright image on a dark background, they required a two-step process, i.e., laser writing of the dark background followed by laser writing the bright pattern to be superposed on the dark background. With the single cell and cell position, the total time (t.sub.t) to photoexpose a number (N) of PWBs would be given by the sum of the time (t.sub.p) to write the wiring pattern on the cell and the number of boards (N) times the sum of the board alignment and change time (t.sub.a) and exposure times (t.sub.e). Thus, EQU t.sub.t =t.sub.p +N(t.sub.a +t.sub.e).
For a pattern of typical complexity and current laser liquid crystal writing system technology, a typical t.sub.p may be about 450 seconds, a typical t.sub.e +t.sub.a may be about 15 seconds. Thus, if fewer than 30 boards of a type are to be exposed, over half of t.sub.t is due to the cell pattern write time t.sub.p. The industry trend is to use smaller board lot sizes. Typical throughput of a modern PWB production facility's automated line may be about 4 boards/minute. Thus, the typical times stated above would slow the production rate by a factor of 2 for a lot size N of 30 boards. This would be unacceptable.
In order to write bright-on-dark patterns, t.sub.p would become even larger, and PWB production throughput reduced further because it could take comparable times to write the dark background with the laser and to write the final bright-on-dark pattern. Thus, the typical pattern generation time would become 15 minutes rather than 7.5 minutes.
Prior art systems for fabrication of printed wiring boards used transmission cells or masks because it was believed that higher contrast ratios were achievable with transmission cells than with reflection cells. Although U.S. Pat. No. 4,013,466 issued to Klaiber at Western Electric in March, 1977 disclosed a method of preparing a circuit by utilizing a liquid crystal artwork master, Heinz and Oehrle also of Western Electric indicated in their paper in the April, 1977 issue of the Western Electric Engineer that while direct exposure of printed wiring boards is "theoretically possible", no system with practical (short) exposure time had been realized. In summary, the possibility of exposing PWBs with TALCs was discussed by various workers at Western Electric, but due to fundamental limitations in contrast, exposure time, and writing speed, it was never publicly demonstrated or reduced to practice.